The invention relates to a digital Phase Locked Loop, comprising a Voltage Controlled Oscillator and a phase detector whose output is coupled to an input of the Voltage Controlled Oscillator through a low-pass filter, an output of the Voltage Controlled Oscillator being coupled to a first input of the phase detector whose second input is coupled to an output of a digital oscillator. The invention further relates to a digital oscillator arranged to be used in the digital Phase Locked Loop.
A digital Phase Locked Loop of this type is generally known. The digital oscillator used herein is arranged, for example, as a programmable pulse generator which is also widely known. A programmable pulse generator generates a signal in the form of a plurality of pulses per time unit, for example, 2 million pulses per second. The frequency of this signal is then 2 MHz and may be reduced in a simple manner by blocking one or more pulses per time unit.
When a digital oscillator of this type is used in the Phase Locked Loop (PLL), the signal of the digital oscillator is fed to the phase detector together with the output signal of the Voltage Controlled Oscillator (VCO). Through the low-pass filter the phase detector output signal which represents the phase difference between the two fed signals is presented to the input of the VCO. The time constant of the low-pass filter determines the time required by the PLL for reacting to the frequency changes of the signal of the digital oscillator and is therefore made as small as possible. The lower limit of the time constant is determined by the minimum frequency change of the signal of the digital oscillator. In some digital telephone exchanges a smallest possible frequency change of 1/64 Hz is demanded for a base frequency of 2 MHz with an accuracy of 100 ppm (200 Hz), which change corresponds to a variation of 1/64 pulse per second or a variation of 1 pulse per 64 seconds. The minimum required time constant is in this case 64 seconds, due to which the PLL then exhibits much too slow a reaction to frequency changes.